1. Field of the Invention
The present invention relates to a method for pulling a single crystal and, more particularly, to a method for pulling a single crystal wherein a silicon single crystal or the like used for manufacturing a semiconductor substrate is pulled by the Czochralski method (hereinafter, referred to as the CZ method) or the like so that the density of Oxidation induced Stacking Fault is small while the deformation rate is lowered.
2. Description of the Relevant Art
There are various methods for pulling a silicon single crystal used as a material for forming a semiconductor substrate, and one of them is the CZ method. In the CZ method, a seed crystal held at the front of a pulling axis is brought into contact with the surface of a silicon melt charged to a crucible. Then, by pulling the pulling axis away from the melt at a prescribed speed, the melt is caused to solidify, so that a silicon single crystal whose horizontal sectional form is almost a circle is grown.
However, in the CZ method, the sectional form of a pulled silicon single crystal is liable to variation. Therefore, a wafer cut from such a silicon single crystal having an irregular sectional form tends to be a defective because of its bad form, and the yield tends to be lowered because of a large cutting portion in the periphery of the wafer.
In order to cope with the problem, an apparatus for pulling a single crystal, having an optical detecting means by which the position of a fusion ring formed on the interface of a melt and a pulled silicon single crystal is measured so as to find a diameter of the silicon single crystal, a rotational angle detecting means by which a rotational angle of the silicon single crystal pulled while rotating is detected, and a control means by which the feedback is conducted on the controllable factors such as a pulling speed, a crucible rising speed, and a temperature of the melt using the PID control, has been proposed (Japanese Kokai No. 57-156397). Using the apparatus, the feedback of the diameter at a prescribed angle position makes it possible to hold down a deviation of the single crystal from a true circle, so that an occurrence of local nonuniformity (striation) of the impurity concentration on the surface of the silicon single crystal is inhibited.
One of the items for evaluating the quality of a pulled silicon single crystal is a density of Oxidation induced Stacking Fault (hereinafter, referred to as OSF). The OSF means a stacking fault caused by the precipitation of oxygen, which was dissolved in a silicon single crystal during the pulling thereof, as an oxide in the oxidative heat treatment conducted after the pulling. In order to reduce the OSF density, a method for pulling a single crystal at a high speed has been disclosed (Japanese Kokai No. 01-192795). In the method, a ratio of a pulling speed V to a variation range of the pulling speed d (V/d) is chosen as 1.8 or more, so that the silicon single crystal is pulled at a high speed.
However, the apparatus disclosed in the Japanese Kokai No. 57-156397 was invented without consideration about a reduction of the OSF density. In the pulling of a silicon single crystal using the apparatus, it is difficult to reduce the OSF density therein.
On the other hand, the method disclosed in the Japanese Kokai No. 01-192795 was invented without consideration about an increase of the deviation from a true circle (hereinafter, referred to as deformation rate). Since the horizontal sectional form of the silicon single crystal is liable to differ from a circle, the number of chips collected from a wafer after the pulling tends to decrease, or since it is necessary to make the external shape uniform by cutting the wafer, the yield tends to be lowered. When the performance of an apparatus used for the CZ method, the classification of a pulled single crystal and the like are different, it is difficult to cope with such cases by the method wherein the above set point is used.
The deformation rate is greatly affected by the pulling speed. In addition, a temperature of a melt, a revolutional speed of a crucible or pulling axis, a change on standing of a dissolving apparatus and the like complexly interact with one another. As a result, it is difficult to control the OSF density and the deformation rate at the same time.
As described above, in the conventional methods for pulling a single crystal, when the OSF density is reduced by making the pulling speed higher, there is a possibility that the deformation rate becomes large. As a result, it is inevitable to take a safety factor into consideration, resulting in the pulling of a single crystal at a relatively low speed in a practical manner. Then the OSF density tends to be large, leading to the lower quality of the silicon single crystal, lower productivity, and higher cost.